Nitrate effect in the heterogeneous hydrolysis of dinitrogen pentoxide on aqueous aerosols

The heterogeneous hydrolysis of N2O5 was investigated on NaHSO4, Na2SO4, an d NaNO3 aerosols. The experiments were performed in the large Aerosol Chamb er at FZ Julich at room temperature and ambient pressure for several relati ve humidities. These salts are components of aerosols in the marine and coa stal boundary layer. For the sodium sulfate aerosols at relative humidities of 50-70% the reaction probabilities gamma(N2O5) were in the range of 0.02 -0.04. For NaNO3 aerosol at similar relative humidities we observed gamma(N 2O5) Of 0.0018-0.0032. With increasing relative humidity, i.e. with increas ing dilution of the nitrate concentration in the aerosol droplets, gamma(N2 O5) increases to 0.023 at 90% relative humidity. Our observation of decreas ing gamma(N2O5) with increasing nitrate concentration can be explained with in the framework of an ionic mechanism for the hydrolysis of N2O5, if the r ecombination reaction of NO2+ with NO3- to N2O5 is considered. By a steady state analysis we derived analytical expressions of gamma(N2O5) as a functi on of the nitrate concentration for a reaction either throughout the aeroso l volume or in a thin surface shell. Accordingly, increasing nitrate concen tration should enhance the lifetime of physically dissolved N2O5(aq) and as a consequence the heterogeneous hydrolysis of N2O5 should change from a ne ar-surface to a volume reaction. The observation of such a specific nitrate effect can be regarded as further experimental evidence for the ionic reac tion mechanism in the uptake of N2O5 on aqueous aerosols. A nitrate effect may gain (local) importance in the atmosphere if increasing NOx emissions t ranslate in an increasing nitrate fraction in the secondary aerosol of anth ropogenic origin.